With the progress of information society, amounts of information are growing increasingly. To accommodate the growing amounts of information, an information recording system with dramatically high recording density as well as a recorder/reproducer based on such a system are longed for.
As a recording system which implements high recording density, attention is given to a recording system which produces a minute opening smaller than the wavelength of incident light and forms a beam spot smaller than the wavelength of light using light produced through the opening.
Known minute openings for optical recording systems include an opening provided at a sharp tip of an optical fiber as described, for example, in Patent Document 1. The opening is produced by partially cutting off a sharp tip of an optical fiber coated with a metal film, using a particle beam such as a focused ion beam (FIB).
Another conventional technique disclosed in Patent Document 2 involves a sloped opening produced in a flat plate. Specifically, a pattern is produced on a silicon substrate by lithography technique, a recess shaped like an inverted pyramid is created by anisotropic etching of the pattern, and the vertex of the inverted pyramid which forms the deepest part of the substrate penetrates to the back side of the substrate. Known methods of penetration include a method which involves grinding the back side of the silicon substrate and a method which involves etching.
Also, Non-Patent Literature 1 discloses a method for vapor-depositing a metal on a sharp core tip of an optical fiber to improve light propagation efficiency.
Furthermore, Non-Patent Literature 2 discloses a shape of an optical fiber which improves both beam spot size and propagation efficiency.
An optical head disclosed in Patent Document 3 has a planar structure made of a symmetrical two-dimensional pattern with a tip of the head composed of a highly refractive dielectric material of a trapezoidal shape. The patent document 3 discloses a method for reducing spot diameter using an inclined trapezoidal surface and the planar structure.
(Patent Document 1)
Japanese Patent Laid-Open No. 10-206660
(Patent Document 2)
U.S. Pat. No. 5,689,480
(Patent Document 3)
Japanese Patent Laid-Open No. 2002-188579
(Non-Patent Literature 1)
“Optical Review,” 1998, Vo. 15, No. 6, pp. 369-373
(Non-Patent Literature 2)
“Applied Physics Letters,” Vol. 73, No. 15
However, optical fibers have low usability of light. For example, with an opening of 100 nm, emergent light intensity is not more than 0.001% of incident light intensity. To deal with this situation structures have been proposed such as a structure in which an apical angle changes in steps from the root to the tip of an optical fiber and a structure in which a minute metal ball is formed in the center of the minute opening at the tip. However, methods of forming a minute opening by sharpening a tip of an optical fiber lack uniformity in vapor deposition of a metal film and have a problem of unstable etching speed arising from concentration of etching solutions and material composition of the optical fiber. Furthermore, there are problems accompanying mass production processes, such as instability in the production of sharp circular-conical apical angles of optical fibers and difficulty to control FIB-based cutting of tips. Besides, the use of optical fibers makes it difficult to adopt multiple heads, which is a common means of increasing data transfer speed.
With conventional high-efficiency techniques using optical fibers, the circular-conical shape at the tip surrounded by metal produces lens effect on a circular-conical surface in the optical fiber, causing an electric field in the incident light to concentrate on some location. The technique in Non-Patent Literature 2 reduces beam spot size and improves efficiency by providing an opening in the surface on which the electric field concentrates. Although this method is effective, it requires extremely high machining accuracy and thus has machining problems similar to those described above.
The method which forms a minute opening by etching a semiconductor substrate has problems of instability during fabrication processes, including instability in the etching rate of the opening with a size of tens of nm, instability in the opening size due to nonuniform thickness of the silicon substrate relative to a certain amount of etching, and instability in the shape of etched part due to shifts in crystal orientation during slicing of the semiconductor substrate. Also, the inverted pyramid shape depends on crystal orientation inherent to the semiconductor substrate, and thus it may not be possible to obtain a desired optimum angle. Furthermore, substrates go through a large number of separation and melting processes, consuming materials heavily and resulting in high costs.
In view of the above problems, the optical head proposed in Patent Document 3 has a two-dimensional pattern at its tip, uses a highly refractive material as light propagating material in the head, and thereby reduces the size of a spot on which light and electric field strength concentrate. Also, the optical head has a multilayered structure sandwiching the light propagating material of the two-dimensional pattern, and optical interference among the multiple layers causes light to concentrate on the light propagating material. The two-dimensional pattern and multilayered structure can be created by the application of lithography technique or the like, and thus the optical head can be machined with high accuracy and be easily built integrally with a magnetic sensor head. However, with the conventional structure, it is difficult to design an optical head capable of efficient light propagation to be large enough to allow for manufacture and machining, and there is demand to work out a structure which will provide high light propagation capability even if it is large in size. Also, conventional methods for designing a multilayered structure for such an optical head are inefficient because they involve modifying individual layer thicknesses by checking optical interference among multiple layers via simulation. Thus, a design apparatus capable of more far-sighted design is desired.